Shaping the ferroelectric domains as waveguide, grating, lens, and prism are key to the successful penetration of periodically-poled ferroelectrics on various wavelength conversion applications. The complicated structures are, however, difficult to be fully characterized, especially the unexpected index contrast at the anti-parallel domain boundaries are typical in the order of 10−4 or less. An ultrahigh resolution optical coherence tomography was employed to fully characterize the domain boundary and structure properties of a periodically-poled lithium niobate (PPLN) waveguide with an axial resolution of 0.68 μm, an transversal resolution of 3.2 μm, and an index contrast sensitivity of 4x10−7. The anti-parallel domain uniformity can clearly be seen non-invasively. Dispersion of the ferroelectric material was also obtained from 500 to 750 nm.
©2011 Optical Society of America
Periodic domain reversals in ferroelectric crystals such as lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) have drawn large attention because of its wide-spread applications in wavelength conversion [1–4] and electro-optics . Moreover, the property of domain boundary in ferroelectrics is an attractive issue especially for three-dimensional (3D) observations [6,7]. Ideally, the ordinary index (no) and the extraordinary index (ne) should not be changed at anti-parallel domain boundary. Even so, it has been reported by various groups that ordinary refractive index variation exists near the anti-parallel domain wall, due to internal field difference of adjacent domain or poling-induced strain , which is usually unexpected and can be reduced by annealing process . The defect issue plays a key role on the characteristics at the domain boundary . Knowledge of its density and distribution provides information of material composition and poling mechanism, and can be served as indicator of the quality of fabricated devices. The unexpected index difference near domain boundary can be observed in LiNbO3 and LiTaO3 with congruent composition [Li/(Li + Nb)~0.485] using optical microscopy [11–13] and disappears in near-stoichiometric composition [Li/(Li + Nb,Ta)]~0.5] . However, the previous researches on the refractive index difference near domain boundary were only for ordinary wave (no). The measurement data for extraordinary wave (ne) are not in literature yet.
In this study, we use an ultrahigh resolution optical coherence tomography (OCT) technique to non-invasively profile the extraordinary index (ne) of periodically poled congruent LiNbO3 (CLN) near the domain boundary in three dimensions and the refractive index difference was estimated [15–17]. The 3D information shows the uniformity of domain boundary and the poled completeness of domain in whole cross section. In addition, the dispersion of a MgO-doped congruent LiNbO3 (MgO:CLN) was characterized from the broadband OCT signal. The axial resolution down to submicron order in high refractive index (n~2.2) material makes it suitable for nondestructive examination on poled periods with microstructures, such as waveguide, grating, lens, and prism.
2. Experimental setup
The OCT system light source was based on a Ce3+:YAG double-clad fiber (DCF) fabricated by a co-drawing laser-heated pedestal growth (CDLHPG) method. The core and inner-clad sizes were accordingly 10 and 100 μm. The DCF was pumped by a 300-mW, 446-nm blue laser diode and the excited visible amplified spontaneous emission (ASE) power was 500 μW. The central wavelength λ0 and the FWHM Δλ of Ce3+:YAG were 560 and 98 nm, respectively. Due to the near Gaussian spectral shape, low image pixel cross talk was expected. To probe the birefringence, the OCT system was configured as polarization-analytic detection. The axial and transversal resolutions were 0.68 and 3.2 μm in the PPLN crystal, and the depth of focus was about 126 μm.
3. Experimental results and discussions
3.1 Etched surface morphology
As a preliminary test on the system resolutions, HF was used to etch the crystalline surface. The periodical domains were distinguished by different etching speeds at the + z and -z faces. The etched surface was scanned by both the OCT and an atomic force microscope (AFM) to compare the etched width and depth as shown in Fig. 1 . The etched depth between domain boundaries was 307 ± 63 nm by the OCT and 310.6 ± 9.3 nm by the AFM. Compared with AFM, the advantage of OCT comes from the capability of interior probing non-invasively.
3.2 Domain boundaries analysis
Figure 2 shows an axial scan by the OCT without any HF etching treatment. A series of peaks after Hilbert transformation can be observed inside the crystal due to the changes of refractive index at the domain boundaries. The signal reflected from the first domain boundary (peak 2) was merged with the wide signal envelope of peak 1, which was generated from the air-crystal interface. The inset shows the etched pattern from the z-face, which indicates the corresponding relation to the interferometric signals. With the current system’s signal-to-noise ratio, such a result can only be obtained when the polarization of light is set along the c-axis (E-field parallel to the c-axis). The cross-sectional image was scanned as shown in Fig. 3 .
The magnitude of refractive index difference can be estimated according to a multiple reflection scheme. Assuming the absorption of LiNbO3 is negligible because of the short-range scanning, the peak value of each interference signal is proportional to the square of the reflected light intensity from the sample arm.
Considering an incident light (I0) from the air (n0 ~1), the refractive index difference (Δn) between the poled and non-poled regions can be estimated using the reflected light from each boundaries,
Ii is intensity of the reflected light from the ith boundary. T and R are the transmittance and reflectance at the domain boundaries, respectively. Noting that Δn is much smaller than the bulk index, so R is nearly independent of the sign of Δn.
At a signal to noise ratio of 77 dB for the present polarization sensitive system, the index contrast sensitivity can readily be determined to be about 4x10−7 from the reflectance equation above. I1 and I5~I9 were taken individually into account to calculate the refraction index to avoid the side lobe problem. The refraction index difference Δne is estimated to be 4.2x10−4. Figure 4 shows a 3D image of the domain boundary with 200 μm x 200 μm cross-section. The 3D image shows uniformity of the domain boundary and reveals that the domains were completely poled in the whole cross section.
3.3 Dispersion characterization
The broadening of the peaks in the axial scan resulted from dispersion of the device under test. Using Wiener-Khinchin theorem on the peaks, the dispersion of extraordinary wave of a 5-mol.% MgO-doped congruent LiNbO3 was characterized as shown in Fig. 5 . The dispersion was estimated by two adjacent peaks. The result agrees quite well with the Sellmeier equation [18,19]. Due to the broadband nature of the light source, the dispersion in a wavelength range from 0.5 to 0.7 μm was obtained.
3.4 Axial scan on other ferroelectrics
The axial scan was also attempted on periodically poled LiTaO3 crystals as shown in Fig. 6 . However, the interference peaks from the congruent LiTaO3 were much smaller than those of congruent LiNbO3, which can be attributed to the smaller refractive index difference between anti-parallel domains due to material property . Thus, the deeper signals in domain boundary were difficult to detect. The refraction index difference can be only roughly estimated to be around 5.5x10−4. Besides, there are numerous polarization reversal properties (e.g. optical birefringence, coercive fields, domain stabilization time, domain back-switching and internal fields) that are different in congruent and near-stoichiometric crystals . Similar result occurred in the periodically poled near-stoichiometric LiTaO3. The interference peaks come from the anti-parallel domain boundary is very weak, and only one peak can be observed as shown in Fig. 6.
3.5 Planar ridge PPLN waveguide analysis
For a ridge-type waveguide, only the ridge width and length can be well defined by mask design. However, the waveguide thickness uniformity is crucial for the performance of the device [20,21]. Figure 7 shows the end view and top view of a planar ridge waveguide by an optical microscope. A cleaved face is shown in Fig. 7(a). The top layer is a periodically poled MgO:LiNbO3 ridge waveguide with a 6.75-μm poling pitch and a polished surface (1); the middle layer is epoxy and the bottom layer is an undoped LiNbO3 single crystal substrate. In Fig. 7(b), the waveguide width and the poled period can be indistinctively observed from the etched pattern. Due to the etched surfaces (2) and (3) are facing downward and buried in epoxy.
To probe the waveguide, an OCT incident light (solid yellow line in Fig. 7(a)) enters the polished surface (1) and passes through the ridge for an axial scan. As shown in Fig. 8(a) , a ridge thickness of 4.73 μm is precisely defined by peaks (1) and (3). The thickness of the thin PPLN layer nearby the ridge waveguide can also be measured (dotted yellow line in Fig. 7(a)). The thickness is only 1.3 μm as shown in Fig. 8(b).
A series of axial scans were performed to measure the waveguide thickness. Table 1 summarizes the measurement of thickness and standard deviation of WG-1. We also measure another waveguide (WG-2) on the same substrate with the same fabrication process. Slight variation of thickness toward the rear region was detected from the measurement.
A 3D tomography of WG-1 was obtained as shown in Fig. 9 . The periodic height variation of interface (4) is due to accumulated optical path length of the top layers. The uniformity of layer thickness and roughness of each interface can be observed. As shown in Fig. 9(b), the periodical patterns from (2) and (3) in Fig. 7(a) are enhanced by image processing and the 6.75-μm poled period can clearly be seen. With the non-invasive 3D profiling technique, the poling quality in the waveguiding region as well as the mode matching design at the input and output ports were verified.
It is demonstrated that the complex structure, dispersion, and small index contrast of periodically poled ferroelectric waveguide can be non-invasively characterized. An axial resolution of 0.68 μm, an transversal resolution of 3.2 μm, and an index contrast sensitivity of 4x10−7 were achieved. The index difference between the + z and –z domains in a MgO-doped congruent LiNbO3 was estimated to be 4.2x10−4, which is an important indicator for the quality of the poled ferroelectrics. The high spatial resolution and high index contrast sensitivity technique can facilitate the development of quasi-phased nonlinear waveguide devices for improving wavelength conversion efficiency as well as reducing insertion loss by mode-matched coupling.
This work is partially supported by the National Science Council, Taiwan.
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